Ultra-pure water containing only trace amounts of electrolytes, minute particles, organic substances (to be referred to as TOC hereinafter) has been used in semiconductor plants, nuclear power plants and pharmaceutical plants. As a system for producing such ultra-pure water, a combination of an ion-exchange unit packed with an ion-exchange resin and a reverse osmosis unit has been used widely. With such system incorporating an ion-exchange resin, however, TOC elating from the ion-exchange resin becomes so big in amount that in many cases it can not be eliminated completely by the subsequent unit on reverse osmosis.
As a means to meet such problem, the present inventors proposed a combination of an electrodialysis unit packed with ion-exchange membranes selectively permeable to monovalent cations and a reverse osmosis unit (Japanese Patent Application No. Hei-4(1992)-25180).
This system practically arrests elution of TOC: the unit on electro-dialysis removes monovalent cations such as Na+ or K+ even at extremely low concentrations, and the unit on reverse osmosis is excellent in eliminating polyvalent ions. Therefore, this system allows stable production of ultra-pure water of an extremely high purity.
However, when a variety of crude waters were treated with the system described above, it was found that elimination of TOC by the electrodialysis unit varies greatly according to the quality of the water treated, and does not proceed as expected for certain waters. More precisely, while a water sample is treated, water molecules are cleaved in association with desaltination. Because this cleavage of water molecules might affect the ion-exchange membrane, divalent cations are allowed intentionally to remain in the water, to prevent their damaging effects to the ion-exchange membrane. Contrary to expectation, however, this maneuver allows anions to remain in the water, and the amount of the anions corresponds in electric charges to those of cations to be kept intentionally in the water, especially of divalent cations such as Ca.sup.2 + and Mg.sup.2 +. How much and what kind of anions remain in the water during such treatment vary according to the ionic balance or pH of the original water sample, but roughly speaking, inorganic carbonate ions tend to remain in the water. Among such inorganic carbonate ions, HCO.sub.3 -- (bicarbonate ion) more tends to remain in the water because it can not be readily captured by the subsequent reverse osmosis unit. Thus, with this system the quality of water falls rather near the outlet.
To solve this problem, the present inventors have tried hard to develop a ultra-pure water producing system that allows production of water practically free not only from TOC but also from cations and anions by combining an electrodialysis unit capable of thoroughly removing monovalent cations and anions, and a reverse osmosis unit capable of removing polyvalent cations and anions, and further to confer the system a property which dispenses with such a maintenance work as the renewal treatment of ion-exchange resins with HCl or NaOH, that is, a necessary maintenance work indispensable to conventional systems.
As a result, the inventors advanced a step beyond the previous system which depended on the use of an electrodialysis unit only with membranes selectively permeable to monovalent cations, developed a system wherein another set of membranes selectively permeable to monovalent anions is added to above-described membranes, and found that this system could solve the problem. This finding led to the present proposal of this invention.
The present invention, being based on the finding described above, aims at providing a system for producing ultra-pure water which will allow thorough elimination of not only electrolytes, minute particles and organic substances from the crude water submitted, but also monovalent cations and anions, and divalent and polyvalent cations and anions therefrom.